1. Field of the Invention
The present invention relates to a distributed input-distributed output wireless communication system using a space-time coding technique. More particularly, the invention relates to a method of adaptively allocating transmission power for beam-forming combined with orthogonal space time block codes in a distributed wireless communication system.
2. Description of the Related Art
A relatively new development in wireless communication technologies includes a spatial multiplexing technology and a space-time coding technology. One particular type of the space-time coding is referred to as MIMO (Multiple Input Multiple Output), and allows a plurality of independent radio waves to be transmitted within the same frequency range at the same time by using multiple antennas in transmitting/receiving signals.
Fundamentally, MIMO technology is based on the use of a spatially distributed antenna and the generation of parallel spatial data streams within a common frequency band. The radio waves are transmitted in such a manner that although the individual signals transmitted have the same frequency, they are separated and demodulated in a receiver so as to produce a plurality of statistically independent (i.e., efficiently separate) communication channels. Accordingly, in contrast to a standard wireless communication system which inhibits a multi-path signal (i.e., a plurality of signals of the same frequency which is delayed in time and modified in amplitude and phase), the MIMO may depend on an almost non-correlative (or weakly correlative) multi-path signal in order to achieve an improved signal-to-noise ratio and a higher throughput within an appropriate frequency band.
One of the issues that is to be considered has to do with fading (i.e. fading channels). Fading channels refer to mathematical models for distortion that is experienced by a telecommunication signal over different types of propagation media. Mobile terminals are susceptible to momentary signal loss (fading) that can be corrected by moving the mobile terminal a short distance. The fading is caused by destructive interference often caused by multiple reflected copies of the signal with slightly different phases. In the past, it has been known to combat fading with multiple versions of the same signal that are received and combined, often with multiple antennas, a practice known as diversity.
In one specific application related to the MIMO type technology, a theoretical result presented from the following reference documents [1] and [2] (listed at the last page of the specification) below proved that a distributed antenna (DA) is more profitable than a co-located multiple input multiple output (C-MIMO) channel in terms of capacity. However, more thorough research has not yet been devoted to a method for obtaining enough advantages of the DA in terms of a capacity. With regard to creating a method of utilizing distributed antennas, there has been previously proposed a concept for a distributed wireless communication system (DWCS) in reference document [3] cited herein below, discussing that it is possible to process a transmit and receive signal together in the system, which can result in a large increase of system capacity.
In the DWCS, combining beam-forming with an orthogonal space time block codes (OSTBC) as disclosed in reference document [4] cited herein below, a very encouraging result is produced because of macro-diversity (from the DA), transmission/reception densities (from the OSTBC) and employment of an array gain (from the beam-forming). The same large-scale fading (i.e., shadowing and path-loss) scenario has been chiefly considered in the previous research related to this scheme. However, such large-scale fading, which occurs in a actual system, from a mobile terminal (MT) to the geographically distributed DA, may be greatly different than the research results, which implies that it is uneconomical to equally allocate the transmission power. Therefore, it is indispensable to allocate the transmission power based on channel state information (CSI).
FIG. 1 is a block diagram illustrating a basic construction of a transmission apparatus wherein the orthogonal space time block codes (OSTBC) and beam-forming are combined in a conventional distributed wireless communication system. Data symbols that are to be transmitted by a transmitting party are modulated in a predetermined modulation scheme (not shown) and are input into a space-time encoder 100 so as to be space-time encoded. Thereafter, the encoded data symbols are presented to a plurality of geographically distributed sub-arrays 111, and beam-forming is performed for the respective symbols. As illustrated in FIG. 1, transmission method for the OSTBC and beam-forming has been conventionally adopted on the following assumptions:
1) A base station antenna is separated among a part of the geographically distributed sub-arrays 111 for beam-forming. It is assumed that large-scale fading from a mobile terminal (MT) 120 to each sub-array 111 is the same.
2) A half-wavelength spacing is applied to antennas in each of the sub-arrays 111. There exists a single path from an MT 120 to each sub-array 111, and channels viewed by the antennas in given sub-arrays 111 are perfectly correlated by means of an array response vector. The array response vector can be estimated according to, for example, a feedback or other direction-of-arrival (DOA) estimation scheme.
3) A corresponding normalized array response vector is directly used as a beam-forming weight vector of a jth sub-array 111. A perfect beam-forming is assumed for the jth sub-array 111 so as to obtain the maximum aperture gain “qj” (wherein the aperture gain may be regarded as an average increase of a signal power in a desired direction, which is achieved by the antenna array for one antenna, wherein an assumption is made that the overall transmission power in both the systems is the same).
4) The sub-arrays 111 are sufficiently separated from each other such that some degree of independent fading can occur in signals of each of the sub-arrays 111. In this case, each of the sub-arrays 111 may be considered to be an equivalent transmission unit.
Accordingly, transmission diversity can be obtained by applying the OSTBC to an equivalent unit. An equal power allocation scheme is adopted.
5) The channel is assumed to comprise a quasi-static flat Rayleigh fading channel.
6) The system is completely synchronized.
7) There exists only a single receiving antenna.
However, the following problems are known with regard to the conventional transmission method for the OSTBC and beam-forming, as shown in FIG. 1:
1) The Rayleigh fading is applied only to a none-line-of-sight (NLOS) communication scenario. However, owing to the distribution of the distributed antennas (DA), there may be some cases where a line-of-sight (LOS) signal exists in the DWCS. As a Nakagami fading is a more general fading model, the Nakagami fading must be considered instead.
2) Large-scale fading from a mobile terminal (MT) to the geographically distributed DA may be varied in actual systems as opposed to theoretical or ideal systems. Therefore, the equal power allocation scheme may result in an unacceptable level of performance. So, the transmission power must be optimally allocated in actual systems.
3) It is assumed that the number of antennas in each sub-array is the same. However, in an actual system, it would be necessary to adaptively configure the number of antennas of each sub-array in accordance with communication environments.